Linear alternating-current amplifier

ABSTRACT

A linear alternating-current amplifier with push-pull-connected first and second output transistors Q 1 , Q 2  includes a pilot transistor Q 4  and a control transistor Q 3  which drives the first output transistor Q 1  into saturation when the resistance of the pilot transistor Q 4  is high. The control transistor Q 3  has its base connected to the collector of the pilot transistor Q 4  and its emitter connected to the junction of the emitter of the first output transistor Q 1  with the collector of the second output transistor Q 2 , the latter connection including a biasing resistor R in series with a constant-current generator G (or an equivalent resistor R G  or fixedly biased transistor Q G ) which maintains the potential difference between the base and the emitter of the control transistor Q 3  at its conduction threshold during saturation of the pilot transistor Q 4  and the second output transistor Q 2 .

FIELD OF THE INVENTION

My present invention relates to a linear alternating-current amplifier of the type having a final stage with two cascaded output transistors operating in class AB and driven in push-pull by a pilot transistor operating in class A.

BACKGROUND OF THE INVENTION

In an amplifier of this type, e.g. as disclosed in my copending application Ser. No. 024,077 filed Mar. 26, 1979, one of the transistors of the final stage (referred to hereinafter as the first output transistor) has an emitter forming a junction with a collector of the other transistor of that stage (referred to hereinafter as the second output transistor) whose emitter, like that of the associated pilot transistor, is connected to a usually grounded terminal of a direct-current supply having another terminal tied to the collector of the first output transistor. The latter terminal is also connected, via a diode energized in the forward direction and a constant-current source, to the collector of the pilot transistor and, via the forward resistance of another diode, to the collector of a control transistor whose base is connected to that constant-current source and whose emitter is tied to the aforementioned junction. The control transistor drives the first output transistor into maximal conduction when the resistance of the pilot transistor is high, i.e. during alternate half-cycles of an a-c input signal. When the pilot transistor saturates during the remaining half-cycles of that input signal, the second output transistor is also driven into saturation.

The current/voltage characteristic of such an a-c amplifier is nonlinear for load currents of a magnitude below a so-called quiescent current.

The first output transistor may be completely cut off or conduct at a level below its quiescent current during the operating half-cycles of the second output transistor, thus giving rise to a distortion of the load current at the beginning of its own operating cycle. This distortion, which involves the generation of higher harmonics liable to interfere with nearby high-frequency equipment, is due to the delaying effect of capacitances in the driving circuit of the first output transistor, e.g. the equivalent capacitance of a diode inserted in the base/emitter circuit of the control transistor. Such a diode, designed to stabilize the amplifier against spontaneous oscillations as needed especially in integrated circuitry, lies in parallel with the base/emitter path of the control transistor whose own equivalent capacitance is thus added to that of the diode.

OBJECT OF THE INVENTION

The object of my present invention, accordingly, is to provide means in such an a-c amplifier for suppressing the aforedescribed distortion.

SUMMARY OF THE INVENTION

I realize this object, in accordance with my present invention, by the provision of a biasing resistor inserted in the connection between the emitter of the control transistor and the junction of the cascaded output transistors, together with a source of direct current in series with that biasing resistor for maintaining the potential difference between the base and the emitter of the control transistor at the conduction theshold thereof during the alternate half-cycles in which the second output transistor is rendered operative.

This d-c source is advantageously a constant-current generator with a polarity opposite the flow of emitter current from the control transistor to the junction of the cascaded output transistors. Such a constant-current generator may be represented or approximated by an ancillary transistor with a fixed bias. A simple resistive connection between the biasing resistor and the grounded supply terminal, however, will be sufficient in some instances.

BRIEF DESCRIPTION OF THE DRAWING

The above and other features of my invention will now be described in detail with reference to the accompanying drawing in which:

FIG. 1 is a graph showing a cycle of the load voltage produced by a conventional amplifier of the type here discussed;

FIG. 2 is a similar graph showing an enlarged segment of the curve of FIG. 1;

FIG. 3 is a circuit diagram of the pilot, intermediate and final stages of an amplifier embodying my present improvement; and

FIGS. 4 and 5 are partial circuit diagrams showing certain modifications in the amplifier of FIG. 3.

SPECIFIC DESCRIPTION

In FIG. 3 I have shown an alternating-current amplifier whose final stage comprises two cascaded NPN transistors Q₁ and Q₂ inserted between a high-voltage terminal 3 and a ground terminal 5, the emitter of transistor Q₁ being tied to the collector of transistor Q₂ at a junction point B connected to an output terminal 4. A load shown to comprise a capacitor C_(L) in series with a resistor R_(L) lies between terminals 4 and 5. Transistors Q₁ and Q₂ are energized in push-pull, in response to an input signal V_(in), with the aid of a pilot transistor Q₄, a control transistor Q₃ and two cascaded coupling transistors Q₅, Q₆ as well as a driving transistor Q₇. Pilot transistor Q₄, which is also of NPN type, has its base tied to an input terminal 1 which is coupled via a capacitor C₁ to the signal source V_(in) having a grounded terminal 2; the emitter of transistor Q₄ is likewise grounded. The base of this transistor is biased by way of a resistor R₁ connected to a tap of a voltage divider lying between positive supply voltage V_(S) and ground, this divider consisting of a constant-current generator I₁ in series with a resistor R₂. Another constant-current generator I₂ connected to supply terminal 3 lies in series with a diode D₁ inserted between the bases of NPN transistor Q₃ and PNP transistor Q₅. The latter transistor, whose collector is connected to the base of output transistor Q₂, has its emitter tied to that of NPN transistor Q₆ whose collector is directly connected to supply terminal 3. The collector of control transistor Q₃, tied to the base of PNP driving transistor Q₇, receives positive voltage from supply terminal 3 via the forward resistance of a diode D₅. Such voltage is also applied to the emitter of transistor Q₇ whose collector is connected to the base of output transistor Q₁. The base of transistor Q₆ is tied to a tap of a further voltage divider, consisting of a constant-current generator I₃ in series with diodes D₃ and D₄, inserted between terminal 3 and junction B. That junction is also connected to the emitter of control transistor Q₃ and, through a diode D₂ in series with a resistor R₃, to the base of that control transistor; circuit D₂, R₃ is shown shunted by a virtual capacitance C representing all the parasitic capacitances which lie between the base and the emitter of transistor Q₃.

In the system so far described, the load voltage V_(out) on terminal 4 substantially follows the collector potential of pilot transistor Q₄. When transistors Q₂ and Q₄ both saturate during positive half-cycles of signal voltage V_(in), the cathode of diode D₁ and the junction point B are at the same potential if the two transistors have the same collector/emitter voltage V_(CEsat). With point B directly connected to the emitter of control transistor Q₃ in the conventional manner, the base/emitter potential V_(BE) of transistor Q₃ would then correspond to the voltage drop across the forward resistance of biasing diode D₁ which, it is assumed, is so chosen as to maintain the transistor Q₃ at its conduction threshold while output transistor Q₁ is traversed by the aforementioned quiescent current throughout the operating half-cycle of its companion transistor Q₂. I have found, however, that the bias of transistor Q₃ does not remain constant during this half-cycle but that, owing to such circuit parameters as the base/emitter voltages of coupling transistors Q₅ and Q₆, the saturation voltage V_(CEsat) of output transistor Q₂ is generally higher than that of pilot transistor Q₄ with resulting diminution of the base/emitter voltage V_(out) of transistor Q₃. At the end of that half-cycle, when output transistor Q₂ returns to its quiescent state and the load voltage on terminal 4 rises to its median level, diode D₁ should be effective to render transistor Q₃ conductive; the restoration of the proper initial bias of the transistor base, however, is delayed by the parasitic capacitance C which includes the effective capacitance of diode D₂.

Thus, as illustrated in FIG. 1, voltage V_(out) does not rise sinusoidally at the beginning of the next half-cycle, starting at a time t₁, but undergoes some high-frequency oscillations as more clearly illustrated in full lines in FIG. 2; the next half cycle, beginning at a time t₂, is free from such fluctuations. With the polarities indicated in FIG. 3, load voltage V_(out) goes positive during the operating half-cycle of transistor Q₁ and negative during the operating half-cycle of transistor Q₂.

In order to suppress the oscillations shown in FIGS. 1 and 2, I have inserted in the amplifier of FIG. 3 a biasing resistor R lying between junction B and a point A on the emitter lead of control transistor Q₃. Resistor R is in series with a current source G which may be a constant-current generator similar to those shown at I₁ -I₃. Such a current source may also take the form of a simple resistor R_(G), connected between point A and ground as shown in FIG. 4, or may be a transistor Q_(G) similarly inserted as shown in FIG. 5. The transistor Q_(G) has its base connected to a tap of resistor R₂ but could also be independently biased by a fixed current source and a diode.

The magnitude of the current drawn by source G, whose direction has been indicated by an arrow in FIG. 3, is such as to bias the control transistor Q₃ to its conduction threshold during negative half-cycles of the load voltage V_(out) shown in FIG. 1, i.e. between times t₂ and t₁. Output transistor Q₁, biased into limited conduction by the associated driving transistor Q₇ as long as transistor Q₃ is at the conduction threshold, will therefore become increasingly conductive at the start of the next positive half-cycle of voltage V_(out) so that the undesired voltage fluctuations shown in FIGS. 1 and 2 do not occur. Generator G, resistor R_(G) or transistor Q_(G) may also be regarded as a shunt impedance connected across the series combination of resistor R and transistor Q₂. 

I claim:
 1. In a linear alternating-current amplifier including a supply of direct current with a first and a second terminal, a first output transistor and a second output transistor connected in cascade with each other across said supply, said first output transistor having an emitter which forms a junction with a collector of said second output transistor and further having a collector connected to said first terminal, said second output transistor having an emitter connected to said second terminal, a pilot transistor with an emitter connected to said second terminal and with a collector coupled to a base of said second output transistor for saturating simultaneously therewith during alternate half-cycles of an a-c input signal applied between the emitter and a base of said pilot transistor, said first terminal being connected to a collector of said pilot transistor by an energizing circuit including a constant-current source, and a control transistor with a collector connected to said first terminal, an emitter connected to said junction, and a base connected to said energizing circuit at a point between said constant-current source and the collector of said pilot transistor, said control transistor being coupled to said first output transistor for driving same into maximal conduction during the remaining half-cycles of said input signal,the combination therewith of a biasing resistor inserted in the connection between said junction and the emitter of said control transistor, and a shunt impedance connected across the series combination of said biasing resistor and said second output transistor for maintaining the potential difference between the base and the emitter of said control transistor at the conduction threshold thereof during said alternate half-cycles.
 2. The combination defined in claim 1 wherein said energizing circuit further includes the forward resistance of a biasing diode inserted between said constant-current source and the collector of said pilot transistor, the base of said control transistor being connected to the last-mentioned collector through said biasing diode.
 3. The combination defined in claim 1 or 2 wherein said shunt impedance is a constant-current generator with a polarity opposite the flow of emitter current from said control transistor to said junction.
 4. The combination defined in claim 1 or 2 wherein said source comprises a resistive connection between said biasing resistor and said second terminal.
 5. The combination defined in claim 1 or 2 wherein said shunt impedance comprises a fixedly biased ancillary transistor inserted between said biasing resistor and said second terminal.
 6. The combination defined in claim 1 or 2, further comprising a conductive connection including the forward resistance of a diode between the base and the emitter of said control transistor. 